The genetic controls of early mammalian embroygenesis should be approachable through the study of complex loci, such as those in t-haplotypes, which regulate differentiation. These chromosomes contain a variety of mutations which are embedded in a long region of chromosome, which includes the H-2 complex, in which recombination is suppressed. The embryological effects of lethal t-mutations suggest that they define genes that function sequentially to control switch points at which specific cell lineages become committed to diverge into separate pathways. Genetically, t-lethals appear to be members of a multigene family with related functions. We have recently shown that recombination occurs freely between two complementing t-haplotypes, which suggests that they are mismatched with respect to wild type, either in gene order or DNA sequences, but are structurally similar to one another. This has been confirmed in a preliminary way by findings that the H-2 complex, for example, is transposed or inverted in t-haplotypes. Furthermore, when t-haplotypes are analyzed with H-2 cDNA on Southern blots after digestion with restriction enzymes they are almost identical, whereas normal chromosomes from inbred strains are very different. We will use classical genetic analysis to obtain a detailed map of the many mutant genes trapped in lethal haplotypes. Specifically the position and number of lethal mutations will be determined, and the genes responsible for male transmission distortion and sterility will be identified and mapped. Concurrent cloning and analysis of the structure of t-specific DNA should lead to an understanding of the nature and relationships of the various interacting mutations contained in t-haplotypes. Also, the reason for defective complementation between different lethal t-haplotypes. Also, the reason for defective complementation between different lethal t-haplotypes will be explored in experiments designed to identify deleterious isoalleles. Other experiments will define the extent of several chromosome 17 deletions which will be useful in mapping experiments.